1) Field of the Invention
The present invention relates to a single-to-differential conversion circuit which converts a single signal to a differential signal.
2) Description of the Related Art
Generally, the high frequency circuits realizing receivers in the field of communications such as mobile communications comprise a mixer, a low noise amplifier (LNA), a filter and the like. Most of the LNAs and filters are arranged to have a single output and input and output impedances of 50 or 75 ohms. However, in most cases, the mixers are double balanced mixers since the double balanced mixers are resistant to even order harmonic distortions and leakage of LO (local oscillator) signals. Therefore, the single-to-differential conversion is necessary.
FIG. 9 is a diagram illustrating a first example of conventional single-to-differential conversion circuits. The single-to-differential conversion circuit of FIG. 9 comprises MOS-FETs (metal-oxide semiconductor field effect transistors) 10 and 11, and a constant current source 12.
As illustrated in FIG. 9, an input signal is applied to the gate terminal of the MOS-FET 10. When the gate-source voltage of the MOS-FET 10 varies with the input signal, the output signal OUT+ of the MOS-FET 10 varies with the gate-source voltage of the MOS-FET 10. Since the source terminal of the MOS-FET 10 is connected to the constant current source 12 and the source terminal of the MOS-FET 11, the sum of the source currents output from the MOS-FETs 10 and 11 is constant.
Therefore, when the output signal OUT+ from the MOS-FET 10 increases, the output signal OUT− from the MOS-FET 11 decreases by the same amount as the amount of the increase in the output signal OUT+ from the MOS-FET 10. Conversely, when the output signal OUT+ from the MOS-FET 10 decreases, the output signal OUT− from the MOS-FET 11 increases by the same amount as the amount of the decrease in the output signal OUT+ from the MOS-FET 10. That is, the input signal applied to the gate terminal of the MOS-FET 10 is converted into a differential signal comprised of the output signals OUT+ and OUT−, where the difference in the phase between the output signals OUT+ and OUT− is π.
However, in the single-to-differential conversion circuit of FIG. 9, the signal propagation paths from the signal input point to the current determination points are different. Therefore, the phase difference between the output signal OUT+ from the MOS-FET 10 and the output signal OUT− from the MOS-FET 11 deviates from π.
In addition, since the input impedance of the gate terminal is high, it is necessary to provide a matching circuit in the stage preceding the gate terminal of the MOS-FET 10 when the single-to-differential conversion circuit is used in a 50-ohm system. However, when the matching circuit is provided in the stage preceding the gate terminal of the MOS-FET 10, the matching circuit has frequency selectivity. Therefore, it is difficult to use the single-to-differential conversion circuit of FIG. 9 in a system in which a wide bandwidth is required.
In order to solve the above problem, Barrie Gilbert, the inventor of the Gilbert Cell, proposed a single-to-differential conversion circuit as illustrated in FIG. 10, which is a diagram illustrating the single-to-differential conversion circuit as a second example of the conventional single-to-differential conversion circuits.
The single-to-differential conversion circuit of FIG. 10 is constituted by NPN transistors 20 to 22. When the NPN transistors 20 to 22 are replaced with MOS-FETs, the single-to-differential conversion circuit of FIG. 11 is obtained. Thus, the single-to-differential conversion circuit of FIG. 11 comprises MOS-FETs 30 to 32.
In the MOS-FET 30, the source terminal is grounded, the drain terminal and the gate terminal are connected, and an input signal of the single-to-differential conversion circuit of FIG. 11 is applied to the drain terminal of the MOS-FET 30. The source terminal of the MOS-FET 31 is grounded, the gate terminals of the MOS-FETs 30 and 31 are connected, and an output signal OUT− is obtained from the drain terminal of the MOS-FET 31. The MOS-FETs 30 and 31 constitute a current mirror circuit. The gate terminal of the MOS-FET 32 is grounded, the source terminal of the MOS-FET 32 is connected to the drain terminal of the MOS-FET 30, and another output signal OUT+ is obtained from the drain terminal of the MOS-FET 32.
The operations of the single-to-differential conversion circuit of FIG. 11 are explained below.
When an input signal is supplied to the single-to-differential conversion circuit of FIG. 11, currents having opposite phases flow in the MOS-FETs 30 and 32, respectively. That is, when the input signal is increased, the gate and drain voltages of the MOS-FET 30 are raised, and therefore the drain current of the MOS-FET 30 increases. On the other hand, in this case, the source voltage of the MOS-FET 32 is raised, and therefore the drain current of the MOS-FET 32 decreases.
Since the MOS-FET 30 and the MOS-FET 31 constitute a current mirror circuit, the drain currents of the MOS-FETs 30 and 31 are equalized. Therefore, the phases of the drain current of the MOS-FET 32 as the output signal OUT+ and the drain current of the MOS-FET 31 as the output signal OUT− become opposite. Thus, the single input signal is converted into a differential signal constituted by the above output signals OUT+ and OUT−.
The accuracy of the oppositeness in the output signals OUT+ and OUT− in the single-to-differential conversion circuit of FIG. 11 is higher than that of FIG. 9. In addition, since the input signal is applied to the source terminal of the MOS-FET 32, the input impedance is low. Therefore, the matching circuit is unnecessary, and the frequency characteristics are satisfactory.
However, since the numbers of the MOS-FETs vertically connected on the OUT+ and OUT− sides of the single-to-differential conversion circuit of FIG. 11 are different, the operating points of the MOS-FETs 30 and 31 become different. Therefore, the DC currents are unbalanced.
In addition, for example, when the single-to-differential conversion circuit of FIG. 11 is used as an RF (radio frequency) signal input circuit in a double-balanced mixer, the switching conditions of the MOS-FETs on the OUT+ and OUT− sides of the single-to-differential conversion circuit of FIG. 11 in response to an LO (local oscillator) signal become different since the DC current levels on the OUT+ and OUT− sides of the single-to-differential conversion circuit of FIG. 11 are different. Therefore, signal distortion and LO-signal leakage are likely to occur in the double-balanced mixer.